It is known that secure documents or instruments may be rendered less susceptible to forgery or counterfeiting by including security features in various forms within the body of the document. In fact, the security or integrity of a document or instrument will increase with the number of separate and distinct security features that it employs.
Many security papers and other items of value include a security device or element, such as a security thread, disposed on or within the document. The security device typically includes one or more security features, such as metallic, magnetic and/or luminescent security features, that serve to authenticate the security paper and prevent or deter counterfeiting.
A common type of security thread includes metal-formed characters or indicia disposed on a plastic carrier substrate. Such threads, which are coated with a very thin (e.g., 100 to 500 angstrom) layer of metal, such as aluminum, and then demetallized, display either: discrete metal characters as currently used in United States currency; negative or reverse-image characters as currently used in the new Euro currency; or a repeating pattern of isolated metal blocks containing negative or reverse-image characters as described in U.S. Pat. No. 5,486,022 and as used in Indian and Venezuelan currencies. The characters on fully embedded portions of these threads are visually detectable in transmitted light by members of the public. Moreover, these threads may be detected by conventional thread detectors that detect the presence or absence of conductive features on the threads. The repeating patterns of the threads described in U.S. Pat. No. 5,486,022 and employed in Indian and Venezuelan currencies are machine readable as well as machine detectable.
It has long been recognized that while visually detectable or public security features are both necessary and desirable, the use of machine testable security features offer a heightened level of security.
Magnetic materials have been used as machine testable security features in security threads. Unfortunately, these materials have a degree of inherent color, which renders them visually detectable in reflected light through a surface of a security paper. Attempts have therefore been made to hide or conceal these materials.
U.S. Pat. No. 5,354,099 to Kaule et al., in an attempt to produce an improved security thread that matches the appearance of commercially established metallic threads, provides a means for concealing a magnetic ink. In particular, this reference discloses a security thread that serves to combine magnetics with negative metal-formed indicia (i.e., negative writing) on a plastic carrier film. Kaule et al. teach that a metallic coating, due to its reflective properties, constitutes the outermost layer of the negative writing. The process disclosed in Kaule et al. for forming the negative writing involves the formation of multiple layers on the carrier film and the subsequent removal or detachment of these layers to form the “negative writing”. The subject process relies in part on “mechanically acting treatment methods” to bring out the “negative writing” more clearly. The ability of the process to produce clear and distinct “negative writing” is enhanced when foaming additives are used where such additives cause the ink to increase in volume thereby creating a bulge on the surface of the thread that provides the “mechanically acting treatment methods” with a good point of attack. See Column 5, lines 2 to 7, of Kaule et al.
Unfortunately, the process identified in Kaule et al. is both time-consuming and arduous. Moreover, combining magnetics with the negative writing limits the design options available for the magnetic security feature.
U.S. Pat. No. 6,549,131 to Cote et al. offers an improved method for concealing magnetic materials. By way of this reference, a laminated security device is provided that includes a carrier substrate, a first metallic layer disposed on the carrier substrate, a magnetic layer disposed on the first metallic layer, and a second metallic layer disposed on the magnetic layer. In one embodiment, these layers collectively form graphic indicia on the carrier substrate, either positively or negatively. A preferred method of making the laminated security device of Cote et al. comprises:                a. providing a carrier substrate;        b. applying a metallic layer to at least a portion of one surface of the carrier substrate;        c. applying a magnetic chemical resist containing a heat and/or pressure activatable adhesive to at least a portion of the metallic layer, where the magnetic chemical resist forms a pattern of graphic indicia on the metallic layer,        d. chemically etching the metallic layer to remove exposed portions of the metallic layer, where chemical etching is resisted by the magnetic chemical resist such that the magnetic chemical resist and a portion of the metallic layer underlying the magnetic chemical resist together form the pattern of graphic indicia on the carrier substrate; and        e. applying a foil film, which is provided on a release coated carrier film, to the pattern of graphic indicia on the carrier substrate under heat and pressure, causing the foil film to bond and cover or conceal the magnetic regions of the graphic indicia pattern.        
Although the method of making the laminated security device of Cote et al. offers good process economics, integrating the metal and magnetics into a single graphics design again limits the design options available for the magnetic security feature.
The design of the security thread used in the new Euro currency departs somewhat from the teachings of the prior art references identified above in that instead of using e.g. vacuum deposited metal, these threads employ silver ink to hide the magnetic materials. Unfortunately, the magnetic materials are still visible through an overlying surface of these security papers under reflected light.
A need therefore exists for a security device that overcomes the disadvantages of these prior art security threads.
It is therefore an object of the present invention to provide such a security device.
It is a more particular object to provide a security device that increases the design options for such light absorbing security features.
It is another more particular object to provide a security device that serves to more effectively hide or conceal light absorbing security features, thereby improving the optical quality of security documents that employ this device.
It is yet another more particular object to provide a security device that employs a second carrier substrate, which allows for the use of additional security features or elements such as holographic and/or diffractive security features.
It is a further more particular object to provide a process for preparing such a security device, wherein the process reduces manufacturing demands by eliminating the need to register in one process step and improves process economics by allowing for a reduction in the thicknesses of the layers which make up the device.
It is yet a further more particular object to provide a security document having one or more such security devices at least partially embedded therein and/or mounted thereon, wherein the optical quality of the security document is improved in those areas in which the security device(s) is fully embedded.